1. Field of the Invention
The present invention relates to a developing device for developing a latent image formed on an image carrier and a process cartridge and an image forming apparatus including the same each and more particularly to a developing device using a two-ingredient type developer, i.e., a toner and carrier mixture and a process cartridge and image forming apparatus including the same each.
2. Description of the Background Art
It is a common practice with a copier, printer, facsimile apparatus, multifunction machine or similar electrophotographic image forming apparatus to use a developing device storing a two-ingredient type developer made up of toner particles and magnetic carrier particles with or without additives, as disclosed in, e.g., Japanese patent laid-open publication No. 2000-194194. Today, there is an increasing demand for the size reduction of the developing device of the type described as well as the extension of the life of the developer.
More specifically, the developing device includes a developing roller or developer carrier, two screw or conveying members and a doctor blade. In operation, fresh toner replenished to the developer is circulated by the screws in the lengthwise direction of the device, i.e., in parallel to the axial direction of the developing roller and mixed with the developer thereby. Part of the developer thus mixed is scooped up onto the developing roller, which faces one of the screws, by the developer and a magnetic scoop-up pole formed on the developing roller. The developer on the developing roller is then metered by a doctor blade to an adequate amount. When the developing roller comes to face a photoconductive drum or image carrier, the toner in the developer is transferred to a latent image formed on the drum for thereby developing the latent image. Subsequently, part of the developer on the developing roller moved away from the position where it faces the drum is released from the roller at a position between a magnetic release pole and the scoop-up pole and again returned to the developing device.
On the other hand, Japanese patent laid-open publication No. 2002-40811, for example, proposes to optimize a meter pole and a developer release or peel-off pole formed on a developing roller for the purpose of extending the life of a photoconductive drum and controlling irregularities in image density coincident with the pitch of screws. However, even the scheme taught in the above document cannot insure stable images against aging, as will be described more specifically hereinafter.
When the flux density of a scoop-up pole formed on a developer carrier or developing roller is high, a developer is apt to accumulate around a so-called doctor gap between the developer carrier and a doctor blade although a sufficient amount of developer can be deposited on the developer carrier. The developer, accumulating around the doctor gap, is subject to a heavy physical load and therefore shortened in life due to deterioration. This is particularly true with a small-size developing device in which various structural parts and elements are densely packed.
On the other hand, when the flux density of the scoop-up electrode is low, the developer accumulates around the doctor gap little and is therefore not subject to a heavy physical load, but the developer cannot be deposited on the developer carrier in a sufficient amount. When the amount of the developer deposited on the developer carrier is reduced, irregularities in density are apt to appear in the resulting toner image in accordance with the pitch of the screw or conveyor member facing the developer carrier.
Further, it is necessary that part of the developer deposited on the developer carrier and just moved away from a developing zone be surely released from the developer carrier between the release electrode and the scoop-up electrode. More specifically, the developer passed through the developing zone is unstable in charge condition. If the developer in such an unstable charge condition is not released from the developer carrier, then it is mixed with the other part of the developer scooped up by the scoop-up pole in a desirable charge condition, rendering the charge condition of the entire developer unstable. The resulting developer would form a toner image with an irregular density distribution when again brought to the developing position.
Particularly, in a small-size developing device, a plurality of magnetic poles are density arranged on the developer carrier, so that the developer cannot be surely released from the developer carrier at a position upstream of the scoop-up pole due to the influence of the magnetic force of the scoop-up pole.
Laid-open publication No. 2002-40811 mentioned earlier differs from the configuration stated above in that the developer carrier is rotated in the opposite direction and that the doctor blade is positioned below the developer carrier. This configuration allows a certain amount of developer to be deposited on the developer carrier without resorting to the is it scoop-up electrode on the developer carrier. However, it is not practicable to directly solve the problems ascribable to the size of the flux density of the scoop-up pole.
Technologies relating to the present invention are also disclosed in, e.g., Japanese patent laid-open publication Nos. 06-064396 and 10-142919 and Japanese Patent No. 2,505,814.